This invention relates to a method and device for modulating a light beam. More specifically, this invention relates to a light modulator that is electro-mechanically biased in order to more efficiently modulate optical signals by having an improved gamma response.
For light modulating devices, the relationship between an applied signal (usually a voltage) and the resulting output illumination intensity is referred to as the response function or transfer function. For many devices, this response function approximates the function form of I=kVxcex3, where I is the output illumination intensity, V is the applied voltage, k is an arbitrary constant that is selected for the device under consideration, and xcex3 (gamma), which determines the response of the modulator, is the exponent. Thus, light modulators can be described simply by their xe2x80x9cgammaxe2x80x9d response. A gamma value of 1 describes a light modulator for which the intensity varies linearly with the applied signal. When such a device is used in an image display device, it is difficult to render images without observable granularity or visible xe2x80x9cstepsxe2x80x9d in the darkest portions of an image. Thus, a higher gamma value is desirable for displays. Also, the best image reproduction can be obtained by matching the display device gamma to the manner in which the image was originally recorded, referred to as the source or content gamma. Conventional image source gamma values are in the range of 2 to 3.
What is needed is a light modulator that has a tunable gamma response that can be optimized for a variety of applications.
Embodiments of the present invention include an apparatus having an improved, tunable gamma response. The apparatus comprises a light modulator having a plurality of spaced-apart elements, having alternating active elements and passive elements lying in a first plane; a gamma controller; and a displacement controller. The gamma controller applies a gamma voltage to a substrate, creating an electro-static field that pulls the spaced-apart elements to a bias plane parallel to the first plane and preferably closer to the substrate. In a reflection mode, the plurality of spaced-apart elements all lie in the bias plane, and a light beam impinging on the plurality of spaced-apart elements will be substantially reflected so that the plurality of spaced-apart elements act as a specular mirror. A low-noise optical receiver that accepts only diffracted light (a first-order receiver) will detect a signal having a substantially zero illumination intensity. In a diffraction mode, the displacement controller applies a displacement voltage to the active elements. In response to the displacement controller, the active elements are moved to a second plane substantially parallel to the bias plane and preferably closer to the substrate so that a light beam impinging on the light modulator will be substantially diffracted. A first-order receiver will now detect a signal having a non-zero illumination intensity. The illumination intensity of the detected signal is proportional to the displacement voltage raised to an exponential power known as the gamma response. Preferably, the gamma response is between approximately 1.75 and 3.0.